This invention relates to a sintered alloy suited to be used for a valve seat of internal combustion engines. Particularly, the invention relates to the technique which can improve machinability as well as superb wear resistance at elevated temperatures.
Recently, with the progress of higher performance of automobile engines, the conditions for operation became much harder. The valve seat used for such engines is also inevitably required to withstand severer environments than ever. To fulfill such requirements, the present applicant previously proposed several sintered alloys having superb wear resistance as disclosed, for example, in Japanese Patent Publication Nos. 17968/74, 36242/80, 56547/82, and 55593/93, and Japanese Patent Unexamined Publication Nos. 195012/97, 195013/97, 195014/97, 219417/98, and Japanese Patent Application No. 141976/98.
Of all the proposed sintered alloys having superb wear resistance, the alloy disclosed in Japanese Patent Publication No. 56547/82 is improved in the wear resistance. The sintered alloy having such improved wear resistance is produced by preparing, in order to form a matrix for a valve seat, two kinds of alloy powders by alloying all the components contained in the alloys disclosed in Japanese Patent Publication Nos. 17968/74 and 36242/80 only excepting graphite, then mixing a graphite powder to the alloy powders thus prepared, and then forming and sintering the mixed powder so that a porphyritic matrix structure will be exhibited. Specifically, graphite is added to a mixed powder of A alloy powder consisting, percent by weight, Cr in the amount of 2 to 4%, Mo in the amount of 0.2 to 0.4%, V in the amount of 0.2 to 0.4%, and the balance of Fe, and B alloy powder consisting of, percent by weight, Ni in the amount of 0.5 to 3%, Mo in the amount of 0.5 to 3%, Co in the amount of 5.5 to 7.5%, and the balance of Fe to set the weight ratio between the A alloy powder and the B alloy powder to 25:75 to 75:25. By doing so, the wear resistance is improved.
The sintered alloy having superb wear resistance disclosed in Japanese Patent Publication No. 55593/93 exhibits a metallographic structure in which particle groups consisting of Mo in the amount of 26 to 30%, Cr in the amount of 7 to 9%, Si in the amount of 1.5 to 2.5% and the balance of Co disperse in the alloy disclosed in Japanese Patent Publication No. 36242/80. The sintered alloy having superb wear resistance can be produced using the mixed powder of the above B alloy powder, C alloy powder consisting of Mo in the amount of 26 to 30%, Cr in the amount of 7 to 9%, Si in the amount of 1.5 to 2.5% and the balance of Co, and a graphite powder.
The sintered alloy having superb wear resistance disclosed in Japanese Patent Unexamined Publication No. 195012/97 consists of, as a whole, Ni in the amount of 0.736 to 9.65%; Cu in the amount of 0.736 to 2.895%; Mo in the amount of 0.294 to 0.965%; Cr in the amount of 0.12 to 6.25%, C in the amount of 0.508 to 2.0%. The sintered alloy exhibits a metallographic structure consisting of (1) martensite, (2) bainite surrounding sorbite and/or upper bainite as a core, (3) austenite which is high in Ni concentration (4) a particle group mainly consisting of Cr carbide covered by ferrite which is high in Cr concentration. The sintered alloy can be produced by using a mixed powder of an Fe powder partly diffused with Ni in the amount of 1 to 10%; Cu in the amount of 1 to 3%; Mo in the amount of 0.4 to 1%, and 3 to 25% of an Fe--Cr type alloy powder consisting of Cr in the amount of 4 to 25%; C in the amount of 0.25 to 2.4% and the balance of Fe, and 0.5 to 1.4% of a graphite powder.
The sintered alloy having superb wear resistance disclosed in Japanese Patent Unexamined Publication No. 195013/97 consists of, as a whole, Ni in the amount of 0.736 to 5.79%; Cr in the amount of 0.12 to 6.25%; Mo in the amount of 0.294 to 0.965%; C in the amount of 0.508 to 2.0%. The sintered alloy exhibits a metallographic structure in which the following phases are dispersed in the matrix of bainite or a mixed structure consisting of bainite and sorbite. The phase consists of a hard phase core mainly consisting of Cr carbide, ferrite phase high in Cr concentration, which surrounds the hard phase core, and martensite phase surrounding the ferrite. The sintered alloy can be produced by using a mixed powder of an alloy powder consisting of Ni in the amount of 1 to 6% and Mo in the amount of 0.4 to 1%, and 3 to 25% of an Fe--Cr type alloy powder consisting of Cr in the amount of 4 to 25%; C in the amount of 0.25 to 2.4% and the balance of Fe, and 0.5 to 1.4% of a graphite powder.
The sintered alloy having superb wear resistance disclosed in Japanese Patent Unexamined Publication No. 195014/97 consists of, as a whole, Ni in the amount of 0.736 to 5.79%; Cr in the amount of 0.12 to 6.25%; Mo in the amount of 0.368 to 1.93%; C in the amount of 0.508 to 2.0%. The sintered alloy exhibits a metallographic structure in which the following phases are dispersed in a mixed structure consisting of (1) bainite or mixed structure of bainite and sorbite, (2) martensite, and (3) austenite. The phase consists of a hard phase core mainly consisting of Cr carbide, ferrite phase high in Cr concentration, which surrounds the hard phase core, and martensite surrounding the ferrite. The sintered alloy can be produced by using a mixed powder of an alloy powder in which 1 to 6% of Ni is partly diffused to an alloy powder consisting of Mo in the amount of 0.5 to 2%, and the balance of Fe, 3 to 25% of an Fe--Cr type alloy powder consisting of Cr in the amount of 4 to 25%; C in the amount of 0.25 to 2.4% and the balance of Fe, and 0.5 to 1.4% of a graphite powder.
Further, the sintered alloy having superb wear resistance proposed in Japanese Patent Unexamined Publication No. 219417/98 is based on the alloy disclosed in Japanese Patent Publication No. 56547/82 in which the alloy exhibits a porphyritic metallographic structure. In the sintered alloy, in order to strengthen the matrix, Ni is added thereto, and in addition, the Co base alloy power used in Japanese Patent Publication No. 55593/93 and the Fe--Cr type alloy powder used in 195012/97, 195013/97 and 195014/97 for forming hard phases are added thereto. The sintered alloy consists of, as a whole, Ni in the amount of 1.35 to 19.61%, Cr in the amount of 0.9 to 11.05%, Mo in the amount of 1.44 to 9.09, Co in the amount of 3.6 to 20.05, V in the amount of 0.018 to 0.26%, Si in the amount of 0.1 to 0.75%, C in the amount of 0.35 to 1.5%, and the balance of Fe. The sintered alloy exhibits a metallographic structure in which the following first and second hard phases are dispersed in a mixed structure of martensite, sorbite and austenite. The first hard phase comprises a hard phase as a core mainly consisting of Mo silicide, and a diffused phase including diffused Co surrounding the hard phase. The second hard phase comprises a hard phase as a core mainly consisting of Cr carbide, and a mixture phase of ferrite and austenite surrounding the hard phase.
The sintered alloy having superb wear resistance proposed in Japanese Patent Application No. 141976/98 is characterized in that the Fe--Cr type alloy powder used in 195012/97, 195013/97 and 195014/97 for forming hard phases is added to a matrix which is strengthened by adding a Ni powder to an Fe powder. In the sintered alloy, the amount of austenite in the structure can be normalized by compacting, sintering and optionally performing a sub-zero treatment.
As apparent from the foregoing, the present applicant proposed several sintered alloys having superb wear resistance in order to meet with the requirements of the age. However, the present situation is that the conditions for operation are even increased in severeness as the performance of automobile engines is further improved and that the material, which is superior in wear resistance and in strength at elevated temperatures to the aforementioned sintered alloys, is demanded.